There are conventionally known techniques of forming by the use of boron phosphide (BP) a semiconductor light-emitting device, such as a light-emitting diode (LED) and a laser diode (LD), which is one of boron phosphide semiconductors containing boron (B) and phosphorus (P) as constituent elements (see, U.S. Pat. No. 6,069,021). Conventional boron phosphide light-emitting devices are fabricated using, for example, a stacked layer structure where a boron phosphide layer is formed as a buffer layer on a substrate composed of a silicon single crystal (silicon) (see, U.S. Pat. No. 6,069,021, supra). In recent years, there has been invented a stacked layer structure for semiconductor light-emitting devices, which has a light-emitting part comprising a pn-junction double hetero structure that uses as a clad layer a boron phosphide layer with a wide bandgap (see, Japanese Patent Application No. 2001-158282).
It has been heretofore known that on a silicon substrate, a boron phosphide single crystal layer comprising the same crystal plane as the crystal plane constituting the substrate surface grows. For example, on a {100}-Si single crystal substrate, the surface of which is a {100} crystal plane, a boron phosphide single crystal layer comprising {100} crystal planes stacked in parallel with the substrate surface is known to grow (see, “Semiconductor Technology (First Volume)” by Katsufusa SHONO, 9th imp., page 77, Tokyo University Publishing Association (Jun. 25, 1992)). It is also known that on a silicon substrate, a boron phosphide single crystal layer not containing a twin crystal (twinning) at all can be grown (see, “Semiconductor Technology (First Volume)”, supra, page 98). On the other hand, it is known that a {100}-boron phosphide single crystal layer containing twining can also be obtained (see, “Semiconductor Technology (First Volume)”, supra, pages 99-100).
Conventional techniques disclose that the twinning contained in a boron phosphide layer has a property of relaxing the mismatching ratio between crystal lattices (see, “Semiconductor Technology (First Volume)”, supra, page 100). Accordingly, when a boron phosphide-based semiconductor layer containing twinning is used, this can contribute to the production of an LED having excellent characteristics including high emission intensity, for example. However, as disclosed in conventional techniques, the boron phosphide-based semiconductor layer containing twinning cannot be stably obtained. That is, conditions necessary for producing a boron phosphide-based semiconductor layer stably containing twinning -are not heretofore clarified and therefore a light-emitting device having excellent emission intensity, for example, cannot be stably obtained.
An object of the present invention is to provide a boron phosphide-based semiconductor layer comprising a crystal structure where twinning can be stably incorporated.
Another object of the present invention is to provide a boron phosphide-based semiconductor device improved in its properties by having the device provided with a polycrystalline boron phosphide-based semiconductor layer stably containing twinning and having a specific crystal surface as a twinning plane, and to provide a method of producing the boron phosphide-based semiconductor device.
Still another object of the invention is to provide an LED excellent in efficiency of taking out emitted light toward the outside.
Here, the boron phosphide-based semiconductor layer comprising the crystal structure the present invention aims at is not a conventional boron phosphide-based semiconductor layer comprising a film-form single crystal, but a boron phosphide-based semiconductor comprising a polycrystal that is an aggregate of single crystal entities different in the crystal direction of the twinning interface (twinning plane) (see, “Chemical Crystallography” by C. W. Van, 1st ed., pages 75-76, Baifukan (Jun. 15, 1970)).